解5×6×1/7×6×7×1/8
=5×6×6×7×1/7×1/8
=5×6×6×1/8
=5×3×3×1/2
=45/2.偶，对不起少加了个小括号。（5乘6乘7）分之1加上（6乘7乘8）分之1加上（7乘8乘9）分之1解原式=1/(5×6×7)+1/(6×7×8)+1/(7×8×9)=1/7×[1/(5×6)+1/(6×8)+1/(8×9)]=1/7×1/3×[1/(5×2)+1/(2×8)+1/(8×3)]=1/7×1/3×1/2×[1/(5)+1/(8)+1/(4×3)]=1/42×[24/(120)+15/(120)+10/(120)]=1/42×49/120=1/6×7/120=7/720